Although the practice of orthodontics can be traced back at least to the time of the Egyptian mummies, modern developments in the art began in the 1920's when Dr. Edward Angle developed the first edgewise bracket. Such a bracket is for the purpose of connecting an orthodontic archwire to a tooth, as opposed to simply wrapping wires around the tooth and ligating to an activating archwire as had been the practice previously. Later on the so-called twin bracket was developed by Swain to permit the use of the bracket to apply a greater rotating and torquing force to the tooth. Still later developments included the Lewis gull wing bracket, the Steiner spring wing bracket, and the Lang stiff wing which incorporated a hole for ligating to rotate the tooth.
Brackets for orthodontic use were originally hand made from gold. In the late 1930's brackets machined from stainless steel were introduced. Stainless steel is generally satisfactory as an orthodontic bracket material, but presents numerous problems. First, it has traditionally been necessary to individually machine each bracket. This is costly, and also results in highly angular edge surfaces which are very uncomfortable for the patient. Another difficultly involved the distinctive appearance of stainless steel, which many patients find objectionable.
In an attempt to overcome the foregoing and other difficulties, plastic orthodontic brackets were introduced. Plastic brackets can be fabricated so as to eliminate the angular edges of machined stainless steel brackets, and are therefore more comfortable for the patient. It is also possible to make plastic brackets in almost any desired color, including highly transparent brackets. It has been found in practice, however, that the use of polycarbonate plastic orthodontic brackets presents a different set of problems. First, plastic brackets are too weak to withstand desired torquing stresses, so that breakage and failure are not uncommon. Second, in the environment of the mouth plastic orthodontic brackets tend to rapidly discolor due to stains caused by various foods, tobacco, beverages such as tea and coffee, etc.
The foregoing problems relating to the use of stainless steel and plastic brackets are overcome by the use of ceramic orthodontic brackets which were introduced by Dr. James M. Reynolds in the 1970's. Ceramic brackets do not have angular edges of the type associated with stainless steel brackets, and are therefore very comfortable in use. Ceramic brackets are very strong and fully capable of withstanding the forces that are applied during orthodontic procedures. Also, ceramic brackets do not discolor in use, and therefore overcome the problems associated with plastic brackets. Ceramic brackets can be highly translucent so that the color of the underlying tooth is observed through the bracket material, and in such instances ceramic brackets are nearly invisible to the eye.
Regardless of the particular material selected for the manufacture of orthodontic brackets, it is necessary to ligate or tie the bracket to an archwire extending through an archwire slot formed in the bracket. By means of appropriate and proper ligation, force is applied from the archwire to the bracket and in turn from the bracket to the tooth upon which the bracket is mounted. Such force eventually leads to the repositioning of the tooth to the desired orientation.
Orthodontic ligation has typically been accomplished utilizing stainless steel wire. Notwithstanding its widespread use, the use of stainless steel wire to ligate orthodontic brackets to archwires results in serious difficulties. First, stainless steel wires almost universally exhibit sharp ends which cause significant patient discomfort. Second, stainless steel, even in the form of small diameter wires, is highly visible in the mouth, which means that the use of stainless steel ligatures substantially reduces the aesthetic benefits deriving from the use of translucent ceramic brackets.
In order to overcome the foregoing difficulties with stainless steel ligatures, plastic O-rings have been used to ligate orthodontic brackets to archwires. U.S. Pat. No. 4,038,753, granted to Klein on Aug. 2, 1977, discloses such a plastic O-ring ligature. U.S. Pat. No. 4,217,686, granted to Dragan on Aug. 18, 1980, discloses a ligator useful in conjunction with plastic O-rings to ligate orthodontic brackets to archwires.
Although overcoming the difficulties long since associated with stainless steel ligatures, the use of plastic O-rings to effect orthodontic ligation presents its own set of problems. First, the injection molding process which is utilized in the manufacture of plastic O-rings is incapable of maintaining tolerances sufficiently to assure that O-ring ligatures that are nominally the same size can be relied upon to apply the same force. Additionally, and similarly to plastic orthodontic brackets, plastic O-rings tend to stain and discolor when used within the mouth of a patient.
The present invention comprises a method of and apparatus for orthodontic ligation which overcomes the foregoing and other difficulties long since associated with the prior art. In accordance with a first embodiment of the invention, an orthodontic ligature is formed from a material having a high tensile strength which does not stain or discolor when employed within the mouth of a patient. The ligature has sufficient length to be deployed around the tie wings of an orthodontic bracket and over an archwire extending through the archwire slot of the bracket. One end of the ligature comprises an insertion end and is provided with a series of locking teeth each having a locking surface facing away from the insertion end of the ligature. The opposite end of the ligature comprises a locking end and includes a locking aperture adapted to receive the insertion end of the ligature and to engage the locking surfaces of the teeth thereof to prevent disengagement of the ligature after installation.
In accordance with other aspects of the invention, the ligature is formed from nylon. This is advantageous in that nylon does not discolor or stain when utilized within the human mouth. Also, nylon can be manufactured so as to be highly translucent or the ligature of the present invention can be provided in a plurality of colors, if desired. Another advantage is that nylon is a relatively low-friction material as compared to prior art orthodontic materials.
In accordance with still other aspects of the first embodiment, the ligature comprises an elongate strip having a series of locking teeth at the insertion end and having a locking tube or cylinder at the locking end. The locking tube has an axis extending parallel to that of the ligature itself and is positioned in an offset relationship to the strip comprising the ligature. The locking tube has a locking taper at one end thereof which engages the locking teeth of the insertion end of the ligature to prevent disengagement of the ligature after installation. The locking tube is adapted for positioning within the tie slot of an orthodontic bracket which is advantageous in that it hides the locking tube from view and also maximizes patient comfort.
In accordance with a second embodiment of the invention, a ligature is formed from nylon or other, similar materials and comprises an elongate strip having locking teeth at both ends thereof. The locking teeth are tapered in the direction of the adjacent end of the ligature and have locking surfaces facing in the opposite direction. The ends of the ligature are extended in opposite directions through a small diameter locking tube. The locking teeth of the ligature engage one another and the ends of the locking tube to secure the ligature in place. In a third embodiment of the invention a ligature is also formed from nylon, or other, similar materials. Unlike the first and second embodiments, the ligature of the second embodiment does not employ locking teeth but instead has relatively small diameter ends. In use, the opposite ends of the ligature are extended in opposite directions through a locking sleeve which is initially open and which is crushable under the action of pliers, etc. to secure the ligature in place. Either conventional or specially designed pliers may be utilized in the practice of the third embodiment of the invention.